The invention relates to an apparatus and a method for inspecting a surface of a sample. In particular the invention relates to an apparatus for inspecting a sample surface using a plurality of charged particle beams, such as a multi-beam scanning electron microscope. The invention may be applied to charged particles of any type, such as electrons, positrons, ions and others.
Such an apparatus is for example disclosed in U.S. Pat. No. 7,554,094. This US patent discloses an electron microscope comprising a charged particle source for generating an array of primary electron beams. These primary electron beams pass a field lens. Downstream of this field lens, the primary electron beam path is a converging beam path having a cross-over in an intermediate plane upstream of an objective lens. The objective lens directs the primary charged particle beams from the common cross-over towards the sample surface and focuses the primary charged particle beams into an array of individual spots on the sample surface.
From the individual spots of the array of spots on the sample, secondary electrons emanate from the sample surface. In order to detect these secondary electrons, the secondary electron beam path, comprising the plurality of secondary electron beams, is separated from the primary electron beam path. To separate the secondary electron beam path from the primary electron beam path, the known apparatus comprises a beam splitter arrangement between the field lens and the cross-over. The beam splitter utilizes a magnetic field portion to deflect the primary electron beams by an angle β to the right (as viewed in the traveling direction of the primary electron beams) and to deflect the secondary electron beams by an angle γ to the right (as viewed in the traveling direction of the secondary electron beams). After the beam splitter, the separated secondary electron beams are directed towards a detector.
A disadvantage of this system is that the use of the beam splitter as described in U.S. Pat. No. 7,554,094 deteriorates the imaging quality of the apparatus. In other words, the apparatus as described in U.S. Pat. No. 7,554,094 requires a number of additional electron-optical components to correct stigmatism, distortion and/or dispersion introduced by the magnetic field portion, in order to substantially maintain the imaging quality. Additional electron-optical components are arranged both in the primary electron beam path and in the secondary electron beam path.
It is an object of the present invention to provide a multi charged particle beam apparatus for inspecting a sample surface which provides a new detection arrangement for detecting the secondary electrons.